Recently, Prof. Jianfeng Gu’s group from the School of Materials Science and Engineering, Shanghai Jiaotong University, has made important progress in the ductilization mechanisms of BCC structured refractory high-entropy alloys (RHEAs). They chose TiZrHfNbx alloys as model alloy system. Interestingly, through carefully modulating the content of Nb in the system, a considerable increase in tensile ductility (from 8.76% to 33.21%) is achieved with almost no sacrifice of yield strength which，in another word, has concurred the so-called strength-ductile trade-off. This study, entitled with “Ductilization of single-phase refractory high-entropy alloys via activation of edge dislocation” (DOI: https://doi.org/10.1016/j.actamat.2024.120614) was published in the international alloy material top academic journal “Acta Materialia”.
The Ph.D. student Yiwei Wang, from the School of Materials Science and Engineering, is the leading author. The Associate Professor QuanFeng He and Professor JianFeng Gu of the School of Materials Science and Engineering served as the co-corresponding authors of the paper.
The BCC structured refractory high entropy alloys (RHEAs) attracted great research interests due to their superior high-temperature strength and wide range of engineering applications. However, most of RHEAs are suffered from the intrinsic brittleness and their ductility need to be significantly improved. Despite the technological and scientific importance, there are still lack of systematic works to investigate the physical origin of the ductility of RHEAs.
In this work, the researchers address this issue through a model alloy system TiZrHfNbx. Interestingly, through carefully modulating the content of Nb in the system, a considerable increase in tensile ductility (from 8.76% to 33.21%) is achieved with almost no sacrifice of yield strength which， in another word, has concurred the so-called strength-ductile trade-off.
In this study, TiZrHfNbx (x=0.4, 0.6, 0.8, 1.0) was chosen as the model alloy system, where all alloys exhibit single-phase BCC structures. Multiscale experiments and theoretical analysis exhibited that the limited tensile ductility of TiZrHfNb0.4 alloys is mainly due to the confinement of the internal long-straight screw dislocations to specific slip planes. In sharp contrast, the dislocation structures of TiZrHfNb alloys are rather uniformly distributed in the observed area. More detailed analysis demonstrated that the superior tensile ductility of TiZrHfNb alloy was benefits from the activation of edge dislocations, which is rarely seen in common BCC structural alloys. The atomic-scale physical origin of the different dislocation configurations and behaviors can be attributed to the higher misfit volumes of TiZrHfNb alloys. This study not only reveals a new ductility mechanism, but also provides a new pathway to the design of ductile RHEA by modulating the atomic-scale environment.
Figure 1: (a) The engineering stress-strain curves and (b) the corresponding work-hardening rate curves of TiZrHfNbx alloy system. (c) The summarized fracture strain and yield strength for the TiZrHfNbx alloy system. (d) The yield strength versus fracture strain of present alloy system in comparison with other RHEA systems.
Figure 2: The Inverse Pole Figure (IPF) evaluation of (a1)-(a3) TiZrHfNb0.4 sample and (b1)-(b3) TiZrHfNb sample during deformation. The misorientation distribution of (c1) TiZrHfNb0.4 sample at 3 % strain and (c2) TiZrHfNb sample at 15 % strain. (c3) Average grain rotation angle of each sample at different deformation stage.
Figure 3: Morphologies of dislocations and slip planes of (a) TiZrHfNb0.4 and (c) TiZrHfNb. And characterization of different types of dislocation of (b) TiZrHfNb0.4 and (d) TiZrHfNb via two beam conditions.
Figure 4: (a) The uniaxial tensile stress strain curves of TiZrHfNb0.4 at different strain rate. (b) The stress strain curve of strain rate jump test for TiZrHfNb sample. (b) The linear fitting of strength at different strain rate.
Figure 5: (a1) and (b1) atomic-scale HAADF-STEM images of TiZrHfNb0.4 and TiZrHfNb alloys. (a2) and (b2) are the corresponding von Mises lattice strain. (c) The distribution of von Mises lattice strain TiZrHfNb0.4 and TiZrHfNb alloys. (d) and (e) The atomic-scale elemental mapping and line profiles of atomic fraction of different constituent elements taken from yellow rectangle region in the right corresponding EDS maps of TiZrHfNb0.4 and TiZrHfNb alloy respectively.